This application claims the benefit of Japanese Patent Applications No.2002-294609 filed Oct. 8, 2002 and No.2003-138918 filed May 16, 2003, in the Japanese Patent Office, the disclosures of which are hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to, and more particularly to lens actuators, optical pickup units and optical disk apparatuses, and more particularly to a lens actuator which turns an objective lens about a center axis along a focusing direction, about a center axis along a tracking direction and about a center axis along a tangential direction, an optical is pickup unit which uses such a lens actuator, and an optical disk apparatus having such an optical pickup unit.
2. Description of the Related Art
A description will be given of an example of a conventional lens actuator, that is, an objective lens driving unit for an optical disk apparatus, by referring to FIG. 1. FIG. 1 is a perspective view showing the example of the conventional lens actuator.
This example of the conventional lens actuator shown in FIG. 1 uses a single cylindrical coil as a focusing drive coil and two flat coils as tracking drive coils. An objective lens holder 2 holds an objective lens 1, and the objective lens holder 2 is resiliently supported by four wire springs 4 which are drawn out from a fixed member 3 which is provided on a substrate 7. A single cylindrical focusing drive coil 5 and two flat tracking drive coils 6 are provided around the objective lens holder 2. The focusing drive coil 5 and the two tracking drive coils 6 are wound in mutually perpendicular directions. The wire springs 4 are drawn out from the substrate 7 via the fixed member 3, and also function as conductor lines by being electrically connected to the coils 5 and 6 via solder portions 8 at ends of the wire springs 4. Magnets 10, each supported on a corresponding yoke 9, sandwich the objective lens holder 2, so that magnetic poles of the magnets 10 confront each other. Each magnet 10 is fixed to the corresponding yoke 9 via a gap, so that magnetic flux from the magnets 10 penetrate a driving portion of the coils 5 and 6.
The objective lens holder 2 is moved by applying a control current to each of the coils 5 and 6, and the objective lens 1 can be moved in a focusing direction (Z-axis direction) and in a tracking direction (Y-axis direction).
However, there are demands to further increase the recording density of optical disks. Consequently, as a beam spot of light used is made smaller and a numerical aperture (NA) of the objective lens becomes larger, a tilt tolerance of the objective lens becomes more severe. In the case of the conventional lens actuator shown in FIG. 1 employing a drive motor structure, the tilt that occurs when the objective lens 1 makes focusing and tracking operations introduces a big problem.
In other words, in the conventional optical disks such as the compact disk (CD), the wavelength of a laser beam used by an optical pickup is approximately 780 nm, and a track pitch of the optical disk is approximately 1.6 μm. Since the recording density on such an optical disk is relatively low, an optical inclination of the optical pickup with respect to the optical disk surface, that is, the tilt of the objective lens with respect to the optical disk surface, has a relatively large tolerance. For this reason, if the parts of the optical disk apparatus are manufactured with a precision to satisfy the designed manufacturing tolerance and assembled with a precision to satisfy the designed assembling tolerance, the assembled parts are positioned with a satisfactory precision which would not adversely affect the tilt, and the tilt does not become a problem.
However, in optical disks which have been developed recently, such as a digital versatile disk (DVD), the recording density is considerably increased compared to that of the CD. Hence, the wavelength of the laser beam used by the optical pickup in this case is approximately 650 nm, and the track pitch of the optical disk (DVD) is approximately 0.74 μm. As a result, the tilt of the optical pickup, particularly the objective lens, must be suppressed to approximately one-half that of the CD, and there are demands to satisfy such severe conditions.
For example, a Japanese Laid-Open Patent Application No.2001-34974 proposes an objective lens driving unit in which a center of each of two focusing drive coils in the tracking direction approximately matches a center of a magnetic field distribution in the tracking direction, so that a change in a driving force during a tracking operation is approximately the same for the two focusing drive coils. In addition, by making an absolute value of a moment about an axis in a jitter direction small, the tilt associated with focusing and tracking operations is considerably suppressed to such an extent that would satisfy the severe conditions demanded by high-recording-density optical disks such as the DVD.
On the other hand, an actuator which is used for active control must employ a movable mechanism which reduces, as much as possible, lost motion due to non-linearity or the like. For this reason, a Japanese Patent No.2856176 proposes a spring support structure for supporting a lens actuator which requires high-precision control.
In order to realize portable equipments and the like, there are demands in the field of recording and/or reproducing apparatuses to reduce the size, thickness and weight of the structure. Since the optical pickup is provided within the recording and/or reproducing apparatus, there are thus demands to further reduce the size, thickness and weight of the optical pickup and the actuator structure provided on the optical pickup for moving the objective lens.
In addition to the above described demands to reduce the size, thickness and weight of the recording and/or reproducing apparatus, there are also demands to increase the data processing speed and to reduce the cost of the apparatus. In order to meet such demands, it is necessary to reduce the size and weight of the structure of the movable section of the actuator, and to realize the actuator for active control, which generally is an expensive assembly, by a relatively simple structure. Moreover, the structure must enable easy assembly, and it is essential from the point of view of reducing the cost to take measures so that the characteristics are maintained and the generation of defective parts is prevented.
The above described demands to reduce the size, thickness and weight, increase the data processing speed, reduce the cost, and improve the performance are not only applicable to the field of the recording and/or reproducing apparatus, but also similarly applicable to other fields. In apparatuses related to optical disks, there are demands to realize a high-density recording and reproduction, high-quality recording and reproduction, and high-speed recording and reproduction. As described above, much progress is being made in the field of high-density recording and reproduction, and the progress is expected to accelerate when the wavelength of the laser beam is reduced and the NA of the objective lens is increased. But as a consequence, there is a new problem in that the signal deterioration becomes large with respect to the tilt of the objective lens.
In order to cope with this new problem, it is necessary to carry out an active drive correction in a roll direction which was conventionally for passive use, that is, in a radial tilt direction. However, an actuator which can be driven along three axes, that is, in the focusing direction, the tracking direction and the radial tilt direction, will become expensive because it is already difficult to mass produce the actuators which can be driven along two axes at a low cost.
Accordingly, there is a proposed arrangement which divides the focusing drive motor into two which are connected in parallel, so as to form a focusing drive motor portion and a radial tilt motor portion which are driven in the same phase and opposite phases. But in this case, the phase control of the two motor portions becomes complex. On the other hand, there is another proposed arrangement which provides, in addition to the actuator which can be driven along two axes, a joint member which joins the movable section and the fixed section and functions as a radial tilt actuator. But in this case, it is productionwise and costwise essentially the same as additionally providing an actuator which can be driven along a single axis.
A most easily conceivable structure is to provide additional coils for radial tilt on the actuator which can be driven along two axes. However, this conceivable structure requires magnets to be provided on the fixed section and the coils to be provided on the movable section, and the need to additionally provide such parts makes the assembling process more complex and increases the cost of the apparatus. Furthermore, the need to additionally provide such parts inevitably increases the number of required parts and makes the structure of the movable section complex, and may even deteriorate the characteristics of the actuator.